3                     AIR QUALITY

 

Introduction 

 

3.1               This section presents the potential air quality impacts associated with the construction and operation of the Project.  Air sensitive receivers (ASRs) were identified for this study and the potential construction dust impact and odour impact on these receivers were assessed.  Mitigation measures required to protect the ASRs were recommended, where necessary, to meet the air quality criteria.

 

Environmental Legislation, Policies, Plans, Standards and Criteria

 

3.2               The criteria for evaluating air quality impacts and the guidelines for air quality assessment are laid out in Annex 4 and Annex 12 of the EIAO-TM, respectively.

 

3.3               The Air Pollution Control Ordinance (APCO) provides the statutory authority for controlling air pollutants from a variety of sources. The Hong Kong Air Quality Objectives (AQOs), which stipulate the maximum allowable concentrations over specific periods for typical pollutants, should be met. The relevant AQOs are listed in Table 3.1.

 

Table 3.1    Hong Kong Air Quality Objectives

 

Pollutant

Maximum Average Concentration (µg m-3) (1)

 

1-Hour (2)

8-Hour (3)

24-Hour (3)

Annual (4)

Total Suspended Particulates

-

-

260

80

Respirable Suspended Particulates (5)

-

-

180

55

Sulphur Dioxide

800

-

350

80

Nitrogen Dioxide

300

-

150

80

Carbon Monoxide

30,000

10,000

-

-

(1)     Measured at 298 K and 101.325 kPa.

(2)     Not to be exceeded more than three times per year.

(3)     Not to be exceeded more than once per year.

(4)     Arithmetic mean.

(5)     Suspended particulates in air with a nominal aerodynamic diameter of 10 mm or smaller.

 

3.4               The EIAO-TM also stipulates that the hourly total suspended particulates (TSP) level at sensitive receivers should not exceed 500 µgm-3 (measured at 25°C and one atmosphere) for construction dust impact assessment.

 

3.5               The Air Pollution Control (Construction Dust) Regulation specifies site formation is one of the controlled processes.  Contractors and site agents are required to inform EPD and adopt dust reduction measures to minimize dust emission, while carrying out construction works, to the acceptable level.

 

 

 

Description of the Environment

 

3.6               The Project area is in Tuen Mun.  The air quality in the study area is primary affected by traffic emissions and industrial emissions.  The nearest EPD air monitoring station is in Yuen Long.  According to EPD’s “Guideline on Assessing the ‘TOTAL’ Air Quality Impacts”, the latest five years averages monitoring data should be adopted as the background concentration.  Table 3.2 summarizes the annual average concentrations of the pollutants (NO2, RSP and TSP).

 

Table 3.2    Annual Average Concentrations of Pollutants in the Latest Five Years (Year 2002 - 2006) at Yuen Long Air Quality Monitoring Station

 

Pollutant

Annual Average Concentration in the Latest Five Years (mgm-3)

NO2

60

RSP

62

TSP

100

 

Air Sensitive Receivers

 

3.7               Representative air sensitive receivers (ASRs), which are in the vicinity of the sites, were identified for assessment, in accordance with the criteria stipulated in the EIAO-TM.  The details of the representative ASRs for the air quality assessment are listed in Table 3.3 and its locations are shown in Figure 3.1.

 

Table 3.3    Summary of Representative Air Sensitive Receivers

 

ASR No.

Description

Distance to the nearest Site Boundary (m)

A1

Planned Secondary School

10

A2

Planned Primary School

70

A3

Planned Secondary School

42

A4

Planned Primary School

30

A5

Unicorn Garden

95

 

3.8               The planned schools in the vicinity of the TM54SPS are tentatively in operation at end 2017.  The construction dust impact would be focused on the existing ASRs.

 

Identification of Environmental Impacts

 

Construction Phase

 

3.9               The construction works for the Project are scheduled to commence in early 2011 for completion in early 2014.  The major construction activities for the Project including excavation, steel fixing, concreting, brick work and finishing, mechanical equipment installation, pipeworks, roadworks and landscape works.

 

3.10            Referring to the construction programme, the bulk excavation activities to be completed within 7 months and the amount of soil materials to be excavated would be about 18,200 m3.  Assuming the construction period is 10 hours per day and 26 working days per month, the average volume of excavated materials to be handled per hour would be 10 m3.  The average number of trucks (carrying 5 m3 excavated materials) required on-site would be 2 trucks per hour.  The area of the proposed site is about 2000 m2.  The work site is limited area, all the trucks would be well covered and dust suppression measures stipulated in the Air Pollution Control (Construction Dust) Regulation would be applied and the nearest existing ASRs are located at least 95m away from the site, no adverse dust impact at the existing ASRs would be expected. 

 

3.11            There would have two other projects to be undertaken concurrently within 500m of the study area.  The concurrent activities includes (i) Site 2 site formation and superstructure works under the project “Formation, Roads and Drains in Tuen Mun Area 54 – Phase 1 and 2, Private and Public Rental Housing Development in Tuen Mun Area 54”; and (ii) Village sewerage works under the project “Sewerage Upgrading Works in Tune Mun under Agreement No. CE 38/2006 (DS)”.

 

3.12            Referring to the approved EIA Report of “Formation, Roads and Drains in Tuen Mun Area 54 – Phase 1 and 2, Private and Public Rental Housing Development in Tuen Mun Area 54”, the predicted hourly and daily TSP levels due to construction activities at Sites 2, 3 and 4 at Unicorn Garden (ASR A5), the nearest ASRs to proposed TM54SPS site, would be 242mg/m3 and 131mg/m3 (about half of TSP criteria), respectively.  No adverse dust impact was predicted at ASR A5 due to construction activities at Site 2.  For another current project, “Sewerage Upgrading Works in Tune Mun under Agreement No. CE 38/2006 (DS)”, the major construction activities undertaken in the vicinity of the Unicorn Garden (ASR A5) would be laying of village sewerage pipeline.  As the construction period at any one location (not more than 50m of the sewer to be constructed per time) would be short-term (about one month), the estimated amount of excavated materials to be disposed would be not more than 20m3 on each section.  Therefore there would have 4 trucks travelled per month on each section of site and as such dust impacts is considered as insignificant with the adoption of mitigation measures stipulated in the Air Pollution Control (Construction Dust) Regulation. 

 

3.13            Taking into account the insignificant dust impacts arising from the project “Sewerage Upgrading Works in Tune Mun under Agreement No. CE 38/2006 (DS)” and the project “Formation, Roads and Drains in Tuen Mun Area 54 – Phase 1 and 2, Private and Public Rental Housing Development in Tuen Mun Area 54”, and the limited dust impacts from the Project, the predicted hourly and daily TSP levels at ASR A5 may be more than half of the TSP criteria but it would be controlled within acceptable levels with the implementation of dust suppression measures stipulated in the Air Pollution Control (Construction Dust) Regulation. No adverse cumulative dust impacts would be expected at the nearest ASRs such as Unicorn Garden and Kei Lung Wai.

 

Operation Phase

 

3.14            The main potential air quality impact during the operational phase of the project would be odour including hydrogen sulphide gas (H2S) arising from TM54SPS.  There is no other odour emission source within 500m from the project boundary that should be considered for cumulative odour impact assessment. The potential source of odours of TM54SPS would be the wet wells and screen chambers.  Deodourizing unit is therefore considered in the engineering design.  The wet wells and screening chambers would be enclosed and the vented air would be treated in the deodourizing unit before discharging into the atmosphere.   

 

Assessment Methodology

 

Construction Phase

 

3.15            As mentioned in Section 3.9 – 3.13, insignificant dust impact would be expected during construction phase of the Project, therefore, only qualitative assessment would be undertaken in the study.

 

Operation Phase

 

3.16            The odour emission rates of raw sewage at sewage pumping station including wet well and screen chamber in the study were derived from empirical equation developed by Shahalem(2). This equation was established from the laboratory results which indicated that the odour intensity of raw sewage at wet well was found to be a function of sewage temperature and oxidation-reduction potential (ORP). Therefore, it is appropriate to use this equation to determine the odour emission rates of the facilities of the sewage pumping station. The emission rates were calculated based on two relationships: the established relationship between odour concentration and physical factors and the volumetric emission flow rate compared with the rate of ventilation. The equation has been employed under various approved EIA Studies such as odour assessment presented in North Lantau Development Topic Report TR23, Environmental Impact Assessment of the Tung Chung Main Sewage Pumping Station, Final Report and the Final Assessment Report of Outlying Islands Sewerage Stage 1 Phase 1 – EIA Study.  Thus emission rates were calculated based on the following two equations:

 

DF        =      1.6 x (T/10)4.9 x (ORP + 200)-0.59  

E          =      DF x V

 

where    DF    =      Odour concentration expressed as dilution factor, OUm-3

            T      =      Temperature of sewage, Fehrenheit F

            ORP =      Oxidation-reduction potential of sewage, mV

            E      =      Emission rate, OUs-1

            V      =      Ventilation rate, m3/sec

                    =      Displayed air volume in the wet well and screen chamber, m3/Retention time, sec

 

3.17            The sewage characteristics were obtained by the Engineer of the consultant team and agreed with the Client.  The sewage temperature was assumed to be 30oC in the assessment as a worst-case scenario.  The sewage was considered as fresh as the sewage comes from Tuen Mun North SPS via twin 750mm rising mains, the break of the rising main just immediately outside the TM54 SPS.  As the total length is about 1.5km, and the minimum design velocity in the rising main is 1m/s in accordance with Clause 7.1.1 of Sewage Manual Pumping Stations and Rising Mains, the total traveling time would be about 25 minutes.  The fresh sewage ORP of 200mV was therefore adopted in the calculation.  The vent pipe is located at 1m above the roof of the TM54SPS which is 7.1m high, the height of vent pipe is 8.1m above ground level.  The diameter of vent pipe is 0.12 m with exhaust velocity of 7 m/s.  The calculation for the emission factor is presented in Appendix 3.1.

 

3.18            The rural model dispersion option of the ISCST3 Model was used to predict 1-hour average odour concentrations at selected sensitive receivers, based on the topographical nature of the vicinity of TM54SPS.

 

3.19            To convert the 1-hour averages to 3 minute averages, the power-law relationship was applied for different stability classes.  For stability class A to F, multiplying factors of 2.23, 2.23, 1.7, 1.38, 1.31 and 1.31 respectively were applied.  To further convert 3-minutes averages to 5-second averages, a multiplying factor of 10 was applied for those hours with atmospheric stability classes A to B, and a factor of 5 for those hours with stability classes C to F.  This was in accordance with EIA Study Report, Sha Tin Sewage Treatment Work Stage III Extension, 1999.

 

3.20            To simulate the worst-case scenario, 360 predefined separate meteorological conditions were used in the dispersion modeling.  The resolution on the wind direction was set to 1 degree increment and a wind speed conditions of 1ms-1.  The models were tested with Pasquil stability classes B and D for daytime hours and class F for nighttime hours.

 

3.21            The assessment points at the residential receivers and schools were considered to the height of 1.5m, 5m, 10m, 15m and 20m above ground.  

 

Prediction and Evaluation of Environmental Impacts

 

Construction Phase

 

3.22            Due to limited work areas and implementation of dust control and suppression measures stipulated in the Air Pollution Control (Construction Dust) Regulation, adverse dust impact at the ASRs would not be expected from the construction activities of this Project.  No unacceptable cumulative dust impact would be anticipated as mentioned in Section 3.11 – 3.13.

 

Operation Phase

 

3.23            Maximum 5-second average odour levels at the closest ASRs were predicted and are shown in Table 3.4 below.  Results indicated that the representative ASRs would all exceed the EIAO-TM odour criterion.  Mitigation measures are required.  Detailed results are presented in Appendix 3.2.  The predicted unmitigated odour contour at worst hit level (10m above ground) is presented in Figure 3.2. 

 

Table 3.4    Predicted maximum odour levels at ASRs (Unmitigated)

 

ASR

Description

Maximum Odour level (5 seconds average) (ou) at different heights (above ground level)

1.5m

5m

10m

15m

20m

A1

Planned Secondary School

2.7

5.8

46.0

7.3

1.9

A2

Planned Primary School

2.6

2.6

11.5

2.0

1.5

A3

Planned Secondary School

3.6

3.9

21.7

3.6

2.1

A4

Planned Primary School

3.6

4.5

28.4

4.7

2.3

A5

Unicorn Garden

2.7

2.6

11.9

3.0

1.5

                     Note: - the odour criterion is 5 odour units over 5-second average.

 

Mitigation of Adverse Environmental Impact

 

Construction Phase

 

3.24            It is anticipated no adverse construction dust impacts at nearby ASRs.  The requirements of the dust suppression measures stipulated in the Air Pollution Control (Construction Dust) Regulation shall be adhered to during the construction period.  In addition, good site practices and a comprehensive dust monitoring and audit programme are recommended to minimise dust impacts.  Control measures relevant to this Project are listed below:

 

              Skip hoist for material transport should be totally enclosed by impervious sheeting;

              Vehicle washing facilities should be provided at every vehicle exit point;

              The area where vehicle washing takes place and the section of the road between the washing facilities and the exit point should be paved with concrete, bituminous materials or hardcore;

              Where a site boundary adjoins a road, streets or other areas accessible to the public, hoarding of not less than 2.4 m high from ground level should be provided along the entire length except for a site entrance or exit;

              Use of regular watering, with complete coverage, to reduce dust emissions from exposed site surfaces and unpaved roads, particularly during dry weather;

              Side enclosure and covering of any aggregate or dusty material storage piles to reduce emissions.  Where this is not practicable owing to frequent usage, watering shall be applied to aggregate fines;

              Open stockpiles shall be avoided or covered.  Where possible, prevent placing dusty material storage piles near ASRs;

              Tarpaulin covering of all dusty vehicle loads transported to, from and between site locations;

              Every stock of more than 20 bags of cement should be covered entirely by impervious sheeting placed in an area sheltered on the top and the 3 sides;

              Every vehicle should be washed to remove any dusty materials from its body and wheels before leaving the construction sites; and

              Instigation of an environmental monitoring and auditing program to monitor the construction process in order to enforce controls and modify method of work if dusty conditions arise.

 

Operation Phase

 

3.25            In accordance with the unmitigated results, the odour levels at all representative ASRs would exceed the odour criterion.  It is recommended that the air ventilated from the wet wells and screen chambers should be treated by deodorizers (granular activated carbon) with 90% odour removal efficiency before discharge to the atmosphere.  All the treated air would be emitted from the exhaust vent of the ventilation fan.  A standby deodourizer would be provided to cater for breakdown and maintenance of the duty deodourizer.

 

3.26            With the installation of deodourizer, the worst-case 5-second average odour levels at the representative ASRs in the vicinity of TM54SPS would comply with the odour criterion and results are summarized in Table 3.5.  The result details are presented in Appendix 3.2.  The predicted mitigated odour contour at worst hit level (10m above ground) is presented in Figure 3.3.  Results indicated that all representative ASRs in the vicinity of the SPS would comply with the odour criterion of 5 odour units.  It is noted that there is exceedance of the 5 odour units out of the project boundary, however, there is no ASR located within the exceedance zone.

 

Table 3.5    Predicted maximum odour levels at ASRs (Mitigated)

 

ASR

Description

Maximum Odour level (5 seconds average) (ou) at different heights (above ground level)

1.5m

5m

10m

15m

20m

A1

Planned Secondary School

0.3

0.6

4.6

0.7

0.2

A2

Planned Primary School

0.3

0.3

1.2

0.3

0.2

A3

Planned Secondary School

0.4

0.4

2.2

0.4

0.2

A4

Planned Primary School

0.4

0.5

2.8

0.5

0.2

A5

Unicorn Garden

0.3

0.3

1.2

0.3

0.2

                     Note: - the odour criterion is 5 odour units over 5-second average.

 

3.27            The following odour precautionary measures should be implemented during operation of TM54SPS:

 

              The entire TM54SPS; especially wet wells and screening collection areas should be enclosed in a building structure;

              Discharge point of the odour removal system should be directed away from the adjacent sensitive uses as shown in Figure 3.1 and the exhaust air should be at a vertical upward flow direction;

              Screened material from the TM54SPS should be stored in a covered container;

              The transportation of screened material during maintenance should be transported in an enclosed type vehicle and disposed off on the same working day;

              Checking and maintenance of the odour removal system should be implemented at least once every half year to maintain the removal efficiency;

              Odour removal systems, e.g. activated carbon filters, should be provided to reduce the odour emissions; and

              A standby deodourizer would be provided to cater for breakdown and maintenance of the duty deodourizer.

 

3.28             As indicated in the above paragraphs, the air quality at the ASRs has already satisfied the odour requirement stipulated in the EIAO-TM with the installation of the deodourization unit for the proposed development at Site 4A as shown on Figure 3.1.  With a view to further improving the air quality, the Project Proponent would liaise with the relevant government departments to see if the school layout could be designed with a larger buffer distance from the TM54SPS such as the tentative layout as shown in Appendix 3.3.

 

Evaluation of Residual Impacts

 

Construction Phase

 

3.29            With the incorporation of Air Pollution Control (Construction Dust) Regulation and EM&A programme, adverse residual impact would not be expected.

 

Operation Phase

 

3.30            It is predicted that air quality at the ASRs would satisfy with the odour criteria with the installation of deodourizing unit, and adverse residual impact is not expected.  In order to ensure compliance of the odour criterion stipulated in the EIAO-TM, commissioning tests for all deodorization system should be included in the Contract Document.

 

Environmental Monitoring and Audit

 

Construction Phase

 

3.31            With the implementation of mitigation measures stipulated in the Air Pollution Control (Construction Dust) Regulation, no adverse dust levels at ASRs would be expected. Environmental monitoring and audit is recommended to be conducted during construction phase of the Project so as to check whether it is in compliance with legislative requirement. Details of the monitoring and audit programme are presented in EM&A Manual.

 

Operation Phase

 

3.32            The predicted odour levels at the ASRs would comply with the EIAO-TM criterion.  It is recommended that odour monitoring and audit should be carried out during the first year of the operation phase by the Environmental Team and Independent Environmental Checker employed by the Drainage Services Department (DSD) to ensure the compliance of odour criteria at the ASRs.  Details of monitoring programme are presented in the EM&A Manual.

 

Conclusion

 

Construction Phase

 

3.33            With the implementation of mitigation measures specified in the Air Pollution Control (Construction Dust) Regulation, dust nuisance at ASRs would not be expected. An EM&A programme was recommended for the construction phase to ensure that the dust criteria would be satisfied at the ASRs.

 

Operation Phase

 

3.34            No adverse odour impact would be expected at nearby ASRs during operation of TM54SPS with the installation of deodourization unit.  Odour monitoring and audit is proposed during the first year of the operation phase to ensure the compliance of odour criteria at the ASRs.